The present invention relates to a method of etching aluminum oxide Al.sub.2 O.sub.3, especially its sapphire form and also spinels, utilizing an AlN etch resisting maskant.
Sapphire has a combination of properties which make it is desirable material both for use as a semiconductor substrate and for batch fabricated ink jet nozzles. In addition to being a hard material, it is resistant to cracking and chemically resistant to attack by most of the commonplace etchants used in semiconductor fabrication. It is also crystalline, transparent, insulating, and has a relatively high termal conductivity.
Interest in the use of sapphire for semiconductor fabrication has increased in recent years. For example, U.S. Pat. No. 3,743,552 to Charles H. Fa describes a method of isolating semiconductor islands wherein the semiconductor regions are electrically insulated by isolation sidewalls of sapphire substrate. The isolation sidewalls are fashioned in the sapphire substrate by preferentially treating the substrate with heat and phosphorous from a phosphorous doped oxide maskant. The method suffers in that phosphorous etching is not easily controlled and requires an additional etchant to remove the etching product. Also, because of the lack of control of the etching process, holes, for instance, of the dimensions required in ink-jet nozzles are not easily obtained.
Other etchants or polishing agents for sapphire are also known. For instance, U.S. Pat. No. 3,753,775 to Paul Harvey Robinson et al. discloses the use of borax as a polishing agent. A publication to P. F. Becker et al. entitled "Flame Polishing of Flat Oxide Bars," The Science of Ceramic Machinery and Surface Finishing, ed by S. J. Schneider, Jr., and R. W. Rice, National Bureau of Standards Special Publication 348, May 1972, p. 237, and the publication to J. T. A. Pollock, entitled "Continuous Flame Polishing of Sapphire Filament," The Science of Ceramic Machinery and Surface Finishing, ed by S. J. Schneider, Jr., and R. W. Rice, National Bureau of Standards Special Publication 348, May 1972, p. 247, describe a method of polishing sapphire with an oxyhydrogen torch.
Publications to R. W. Rice et al. entitled "The Strength of Gas Polished Sapphire Rutile," The Science of Ceramic Machinery and Surface Finishing, ed by S. J. Schneider, Jr., and R. W. Rice, National Bureau of Standards Special Publication 348, May 1972, p. 267, and to W. A. Schmidt et al. entitled "Preparation of Smooth Crystalline Damage Free, Sapphire Surfaces by Gaseous Etching," The Science of Ceramic Machinery and Surface Finishing, ed by S. J. Schneider, Jr., and R. W. Rice, National Bureau of Standards Special Publication 348, May 1972, p. 259, disclose the polishing of sapphire with helium-freon gases. These prior art polishing techniques are inadequate for providing sapphire insulating sidewalls or for providing inkjet nozzles, because of the lack of teaching of suitable selective etching by the polishing agents.
Batch fabricated arrays of nozzles for use in ink jet printing systems are known in the art. Such arrays have been etched in silicon. For example, U.S. Pat. No. 3,921,916 to Ernest Bassous, issued Nov. 25, 1975, and copending U.S. patent application Ser. no. 543,600, to E. Bassous et al., filed Jan. 23, 1975, teach the use of monocrystalline silicon for batch fabricated ink jets. The latter also refers to prior art ink jet nozzles which are formed by drilling holes in sapphire by mechanical means, or by the use of an electron beam or laser.
Work has been done on aluminum nitride and aluminum at temperatures to 2000.degree. C as, for example, in an article in the Journal of the American Ceramic Society, 42[2], pp. 53-59, entitled "Aluminum Nitride, A Refractory For Aluminum to 2000.degree. C," by G. Long and L. M. Foster. The following are quotes from p. 59 of the article:
"Contrary to the published data, high purity and highly sintered AlN was found to be very stable and extremely chemically inert to acids and bases. Its oxidation resistance was good, and, particularly, its stability in a reducing atmosphere was excellent . . . PA1 The ability of AlN to serve as an inert container for aluminum melts at elevated temperatures should permit more fundamental studies of aluminum and its behavior near its boiling point . . . Aluminum at elevated temperatures is an extremely active metal, and any container for it may have other refractory applications."
Insofar as the effects of H.sub.2 on sapphire, Extended Abstracts, Volume 75-1 of the Electrochemical Society Spring Meeting, Toronto, Canada, May 11-16, 1975, Abstract No. 252 by E. A. Gulbransen and K. F. Andrew, indicates a study made, the objectives of which were, inter alia, to evaluate the effect of the oxygen potential of H.sub.2 /H.sub.2 O gas mixtures on the reduction and volatilization (vapor transport) reactions of Al.sub.2 O.sub.3 at 2000.degree. C. They found that rapid transport reactions were found in dry hydrogen atmospheres.